World wide air traffic is projected to double every ten to fourteen years and the International Civil Aviation Organization (ICAO) forecasts world air travel growth of five percent per annum until the year 2020. Such growth may have an influence on flight performance and may increase the workload of the flight crew. In order to handle the expected increase in air traffic, the Next Generation Air Transportation System (NextGen) will introduce major transformations in Air Traffic Management (ATM), for example, aircraft trajectory-based operations. Trajectory-based operations will manage National Airspace System (NAS) resources by requiring aircraft to precisely follow custom-made 4-D trajectories consisting of a specified path along-path time conformance requirements. This promotes prescribing and accurately following trajectories that ensure separation and optimize traffic flow management over different time horizons, which will significantly improve flight safety and performance.
However, there are three major issues associated with the 4D operational concept encountered during steady wind error environment. First, deconfliction of the airspace and prevention of a separation infringement requires a reliable 4D trajectory. It uses Required Time of Arrival (RTA) to guarantee the reliability of time of arrival at a merging waypoint (RTA waypoint). However, the reliability before the waypoint is not guaranteed. An aircraft not compliant with its 4D trajectory may allow the spacing between aircraft to become too small.
Second, the reliability of the RTA waypoint is limited if the aircraft cannot accelerate/decelerate to compensate for wind and temperature errors. To prevent this situation, aircraft typically use a “reliable RTA window” that is computed as the time at the RTA waypoint in case of maximum speeds and 95% headwind error for a first boundary and minimum speed and 95% tailwind for a second boundary. If for example, the minimum RTA is required, there is still a speed margin of the 95% headwind error that can be used for acceleration. However, if the 95% headwind really occurs, the predictions will expect the wind error to disappear and the aircraft would accelerate less than necessary to compensate for the error. This is desirable to prevent high engine activity, but if the wind error does not disappear, the aircraft will accumulate a position/time error and may saturate the guidance speed far before the RTA waypoint, resulting in the RTA being missed. The issue is symmetrical: the same will happen for the maximum RTA and steady tailwind error. To guarantee reliable RTA at a waypoint 200 NM from the aircraft in case of 95% steady wind error, the margin applied on the speed envelope would need to be about twice the 95% wind error. This is the main weakness of the RTA concept: a limited RTA reliable window size (an aircraft speed envelope at FL350 can be between 20-50 kts based on current gross weight). For a typical 95% wind error of about 8 kts (8 kts true air speed (TAS) equivalent to 5 kts calibrated air speed (CAS) at high altitude), using 10 kts (two times 5 kts) from each side of the speed envelope would drastically reduce the reliable window.
Third, in the case of RTA in descent and steady wind error, the system has to update descent speeds several times, resulting in the previously constructed descent path no longer being optimal. The pilot may have to use airbrakes/thrust to follow or recapture the descent path, resulting in less fuel efficiency, additional pilot workload, reduce passenger comfort, and aircraft structure load constraint that can reduce the residual life of the aircraft structure. From an airline perspective, this functionality may influence the cost of RTA operations. With an improvement in the RTA reliability and the reliable RTA window, an RTA selection close to airline cost & fuel policy may be made.
Accordingly, it is desirable to provide a system and method for improving 4D trajectory reliability, RTA reliability, and descent path and optimization. Furthermore, other desirable features and characteristics of the exemplary embodiments will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.